Hydrogen Chloride Analyzers — TDLAS Line-Selected HCl Measurement

Three properties of incineration flue gas make TDLAS the defensible choice for HCl CEMS: (1) high water-vapor content (typically 10–25 %vol) that overlaps the HCl absorption band in broadband NDIR — TDLAS places the laser line at a specific HCl feature that sits above the H₂O continuum, resolving HCl optically rather than by humidity compensation; (2) the co-presence of SO₂, HF, and CO₂ that introduce spectral background in wider-band techniques, addressed by laser-line selection and application matrix review; (3) the highly corrosive, reactive nature of HCl in wet acid gas at stack temperatures, requiring materials and measurement approaches that preserve sample integrity from probe tip to analyzer cell.

HCl is highly soluble in water. If the sample path temperature drops below the acid dew point (roughly 60–80 °C for typical HCl concentrations in incineration flue gas), HCl dissolves into condensate film on the sample line walls — causing measurement dropout (the HCl reaches the analyzer at a lower concentration than is present at the measurement point) and simultaneous aggressive corrosion of the sample path materials. Standard SS316L is adequate for non-corrosive gases but is attacked by concentrated HCl at elevated temperatures. Hastelloy C-276 — a nickel-molybdenum-chromium alloy — provides markedly better resistance to HCl-bearing acid service at the temperatures and concentrations found at incineration and chlor-alkali installations. The heated line must maintain temperature above the dew point throughout its full length, including bends and connections.

EN 14181 defines the quality assurance framework for automated measuring systems (AMS) installed at stationary source CEMS positions in the EU. QAL1 is the initial type-approval test confirming the instrument meets minimum performance criteria under laboratory and field conditions. QAL2 is the site-specific calibration and uncertainty assessment carried out once the system is installed, using a reference method (typically extractive isokinetic sampling) to establish the functional relationship between the AMS reading and the reference value and to document measurement uncertainty. Annual Surveillance Testing (AST) maintains the QAL2 calibration over time. For HCl CEMS, QAL2 is mandatory for permit compliance under the EU IED — the measurement uncertainty obtained at QAL2 must be within the allowed tolerance for the permit limit, which means instrument selection, installation quality, and sampling system integrity all affect compliance status.

OSHA sets a Ceiling PEL of 5 ppm for hydrogen chloride — workers must not be exposed to concentrations at or above this level at any time. ACGIH sets a TLV-Ceiling (TLV-C) of 2 ppm for the workplace. NIOSH recommends a 5 ppm Ceiling (15-minute) and an IDLH of 50 ppm. For fixed-point continuous workplace safety monitoring, a measurement range of 0–10 ppm HCl with a detection limit below 0.5 ppm is typically specified. These safety-range applications are served by electrochemical HCl detectors as a Conditional path — not the same equipment as incineration CEMS, which operates in a 0–50 / 200 ppm range at stack conditions.

HCl and SO₂ both absorb in the mid- and near-infrared regions, and their spectral features can overlap at some wavelengths in broadband measurement approaches. TDLAS addresses this at the optical level: the laser is tuned to a single, narrow HCl ro-vibrational absorption feature at approximately 1.74 μm in the near-IR overtone band. The SO₂ absorption cross-section at this specific wavelength is substantially lower than the HCl feature, and residual SO₂ background can be accounted for in the application matrix review. Similarly, HF and CO₂ are resolved through line selection and matrix analysis. The matrix review should be conducted for each specific installation, as the magnitudes of SO₂, HF, and CO₂ backgrounds vary between sites and affect the residual interference estimate.

Both the ZS8100-HCl and the sister TDLAS form factor use TDLAS as the measurement principle and share the same fundamental advantage for HCl in wet, corrosive matrices — laser-line-selected non-contact optical measurement with Hastelloy C-276 heated sample path. The difference is platform and form factor: the ZS8100-HCl is on the mainstream GESHINE TDLAS process analyzer platform, while the sister form factor is based on the H₂S-family platform. For most incineration CEMS, chlor-alkali, and PVC/VCM applications, the ZS8100-HCl is the default recommendation. The sister TDLAS form factor is specified when the H₂S-family skid geometry or mounting arrangement fits the project installation constraints better, and is available by project review. Both configurations are confirmed with range and accuracy per project.

Wet-scrubber abatement verification for HCl requires continuous measurement at both the scrubber inlet and outlet. Inlet measurements face the highest HCl concentrations and most corrosive conditions — typically 100–2000 ppm HCl range in incineration flue gas, at high temperature and high moisture. Outlet measurements must detect low-ppm HCl (≤10 ppm for typical abatement targets) against a gas matrix that still carries residual moisture, SO₂, and acid aerosol from incomplete scrubbing. TDLAS at both points, each with its own Hastelloy heated sample line and analyzer, gives the data needed to calculate abatement efficiency in real time and trigger scrubber maintenance when outlet HCl begins to trend upward before the permit limit is reached. The inlet and outlet analyzers can share a DAS or SCADA integration point via Modbus.

MCERTS (UK Environment Agency scheme, administered by Sira) and US EPA Performance Specification 18 (PS-18, Hydrogen Chloride CEMS) are the relevant type-approval frameworks for HCl continuous measurement systems in the UK and US respectively. The ZS8100-HCl is at the MCERTS / PS-18 target scope stage — certification is the intended path for the GESHINE own-brand hardware, but type-approval testing and documentation sign-off have not yet been completed. Until certification is confirmed per project, the analyzer can be deployed for process and non-certified CEMS applications; for regulated permit-CEMS in the UK or US, confirm current certification status at the time of project specification with the GESHINE application team.